Temperature compensating pressure sensor having active and reference membranes

ABSTRACT

Provided is a temperature compensating pressure sensor. The sensor includes a silicon substrate having sealed channels on which is deposited a CMOS layer, and a conductive layer and a passivation layer deposited on the CMOS layer, the conductive layer representing a first electrode. The sensor also includes a conductive active membrane spaced from the conductive layer to form an active chamber, and a conductive reference membrane spaced from the conductive layer to form a sealed reference chamber. Also included is a cap which covers the membranes, said cap having a channel to expose the active membrane to an outside fluid pressure, with the membranes representing a second electrode. The active membrane deflects due to differential stresses so that the first and second electrodes develop a capacitance C between them depending on the electrical permittivity of the fluid, with the reference membrane providing a temperature compensating reference capacitance.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.11/706,305 filed Feb. 15, 2007, which is a continuation of U.S.application Ser. No. 10/965,904 filed Oct. 18, 2004, now issued U.S.Pat. No. 7,234,357, all of which are herein incorporated by reference.

TECHNICAL FIELD

The present invention generally relates to a pressure sensor and inparticular, a micro-electro mechanical (MEMS) pressure sensor.

CO-PENDING APPLICATIONS

Various methods, systems and apparatus relating to the jpresentinvention arer discolosed in the followint co-pending applications filedby the applicant or assignee of the present invention Simultaneouslywith the present application:

7093494 7143652 7089797 7159467 7124643 7121145 7089790 7194901 69687447089798 7240560

The disclosures of these co-pending applications are incorporated hereinby cross-reference.

The following patents or patent applications filed by the applicant orassignee of the present invention are hereby incorporated bycross-reference.

6716666 6949217 6750083 7014451 6777259 6923524 6557978 6991207 67669986967354 6759723 6870259 10/853270 6925875 10/898214

BACKGROUND ART

The invention has wide-ranging application across many fields ofindustry. It is particularly suited to pressure measurement in harsh ordynamic environments that would preclude many other pressure sensors.These applications include, but are not limited to:

-   -   monitoring engine pressure (cars, aircraft, ships, fuel cells)    -   sensors for high speed wind tunnels    -   sensors to monitor explosions    -   sensors for boilers    -   sensors for dish-washing machines    -   sensors for irons (both domestic and industrial)    -   sensors for other steam based machines where overpressure can        lead to destruction and loss of life

However, in the interests of brevity, the invention will be describedwith particular reference to a tire pressure monitor and an associatedmethod of production. It will be appreciated that the Tire PressureMonitoring System (TPMS) described herein is purely illustrative and theinvention has much broader application.

Transportation Recall Enhancement, Accountability and Documentation(TREAD) legislation in the United States seeks to require all U.S. motorvehicles to be fitted with a tire pressure monitoring system (TPMS).This is outlined in U.S. Dept. of Transportation, “Federal Motor VehicleSafety Standards: Tire Pressure Monitoring Systems; Controls andDisplays”, US Federal Register, Vol. 66, No. 144, 2001, pp. 38982-39004.The impetus for this development comes from recent Firestone/FordExplorer incidents which led to a number of fatal accidents. A carefulassessment of tire inflation data found that approximately 35% of in-usetires are under inflated, whilst an assessment of the effect of a TPMSfound that between 50 to 80 fatalities, and 6000 to 10,000 non-fatalinjuries, per annum could possibly be prevented. This is discussed inU.S. Dept. of Transportation, “Tire Pressure Monitoring System,” FMVSSNo. 138,2001. European legislation also appears likely to require thefitting of a TPMS to increase tire life, in an effort to reduce thenumber of tires in use by 60% in the next 20 years, so as to minimisethe environmental impacts.

Two different kinds of TPMS are currently known to be available in themarketplace. One kind of TPMS is based on differences in rotationalspeed of wheels when a tire is low in pressure. The asynchronicity inrotational speed can be detected using a vehicle's anti-braking system(ABS), if present. The second kind of TPMS measures tire pressuredirectly and transmits a signal to a central processor. FIG. 1 (priorart) illustrates a schematic of a typical pressure measurement basedTPMS 10. Sensors 12, provided with a transmitter, measure pressure intires 13 and transmit a signal 14 to antenna 16. The data can then berelayed to a receiver 15 and processed and displayed to a driver of thevehicle 17 on display 18.

Table 1 lists some presently known TPMS manufacturers/providers.Motorola and Pacific Industries have each developed a TPMS, whilst othercompanies listed in Table 1 act as suppliers for TPMS manufacturers,including some automobile producers that install their own TPMS.

TABLE 1 Pressure sensor manufacturers involved in TPMS. Company Supplierto Type of Sensor Motorola Motorola Capacitance Pacific IndustriesPacific Industries Piezoresistive SensoNor Siemens, TRW, Beru,Piezoresistive Porsche, BMW, Ferrari, Mercedes, Toyota Siemens GoodyearPiezoresistive Transense Technologies Under development Surface AcousticWave TRW/Novasensor Smartire, Michelin, Piezoresistive Schrader, Cycloid

There are two main types of pressure sensor; resistive or capacitive.Both types of these sensors rely on deflection of a membrane under anapplied pressure difference. One side of the membrane is exposed tointernal pressure of a tire while the other side of the membrane formsone wall of a sealed cavity filled with gas at a reference pressure.

The resistive-type sensors typically employ silicon-basedmicro-machining to form a Wheatstone bridge with four piezoresistors onone face of the membrane. The sensor responds to stress induced in themembrane. For capacitive-type sensors, the membrane forms one plate of acapacitor. In this case, the sensor responds to deflection induced inthe membrane. Preferably, the responses should be linear with pressure,for predictability, up to at least a critical point.

Transense Technologies, listed in Table 1, have developed a differenttype of sensor, based on surface acoustic wave detection. This sensorrelies on interferometric measurement of the stress-induced deflectionof a reflective membrane. A fibre-optic cable both transmits andreceives laser light, with one end of the fibre-optic cable beinginserted into the interferometer. This system is discussed in Tran, T.A. Miller III, W. V., Murphy, K. A., Vengsarkar, A. M. and Claus, R. O.,“Stablized Extrinsic Fiber Optic Fabry-Perot Sensor for Surface AcousticWave Detection”, Proc. Fiber Optic and Laser Sensors IX, SPIE vol. 1584,pp 178-186, 1991.

Presently, there are also a variety of different kinds of deploymentmeans for sensors in a TPMS, including valve cap and valve stem basedsystems, systems with the sensor mounted on the wheel rim or wheel hub,and also a tire-wheel system developed by an alliance of several tiremanufacturers which has a sensor embedded in the wheel frame itself.These different kinds of deployment in TPMS are listed in Table 2.

TABLE 2 Specifications of TPMS in production. Warning Company/ Type ofLevel Accuracy Group System Fitted to (psi) (psi) Sampling Beru WheelRim Audi, BMW, user set 1 every 3 sec, Mercedes transmitted every 54 secCycloid Wheel Cap Ford, 18 1 30 sec/10 min (pump) Goodyear Fleet ValveCap heavy 20 1 3.5 sec vehicles Johnson Valve Stem AM   19.9 1 15 minMichelin/ PAX Renault, ? ? ? Goodyear/Pirelli/ System Caddillac DunlopMotorola Wheel Rim AM ? ? 6 sec Omron Valve Stem AM ? ? ? PacificIndustries Valve Stem AM 20.3/user   1.8 15 sec/10 min set SchraderValve Stem Corvette, 22 2% ? Peugeot, Cadillac Smartire Wheel Rim Aston?   1.5 6 sec Martin, Lincoln, AM AM = products fitted to a vehicleafter vehicle purchase (After Market).

To increase battery life, most TPMS are in stand-by mode for themajority of time, only operating at set intervals. The U.S. legislationrequires the system to alert the driver within a set time of detectingsignificant tire under-inflation conditions. It also requires a warninglight to signal when the tire is either 20% or 25% under-inflated. Mostof the devices presently available in the market are accurate to within±1 psi, which represents ±3% for a tire pressure of 30 psi. Moregenerally, the sensor should perform in a harsh environment, withtemperatures up to 130° C. and accelerations of 1000 g or more. Tirepressure increases and decreases in response to corresponding changes intemperature. Most systems presently available include a sensor toaccount for thermally induced changes in tire pressure sensorsensitivity (Menini, Ph., Blasquez, G., Pons, P., Douziech, C. Favaro,P. and Dondon, Ph., “Optimization of a BiCMOS Integratetd Transducer forSelf-Compensated Capacitive Pressure Sensor,” Proc. 6^(th) IEEE Int.Conf. Electronics, Circuits and Systems, Vol 2, pp. 1059-1063, 1999).

Fabricating a MEMS pressure sensor on, and through, a silicon wafersubstrate, requires etching from both sides of the wafer. The referencechamber needs to be filled with sacrificial material so that themembrane can be deposited on top. Then, the final etches removes thesacrificial material in the chamber with a deep back etch (etching fromthe reverse side of the wafer through to the front side). The holes fromthe deep etch need to be closed to seal the chamber. Typically, this isdone by securing another wafer to the backside of the main wafer.

Resistive and capacitative pressure sensors rely on the reference fluidpressure remaining constant. If the reference fluid leaks, the sensorsuffers calibration drift. The permeability of the seal closing thebackside of the wafer is the main cause of leakage. It is a simplematter to adhere a separate wafer to the backside of the main wafer inorder to seal the etch holes. However, the air permeability of polymericadhesive would result in the reference cavity pressure equilibratingwith the tire pressure in about 100 days. This is clearly unsuitable.

Another option is to over-mold the sensor with plastic to seal thereference chamber. Plastic has a permeability 10 times less thanadhesive but it will appreciated that this is still too high. Thecalibration drift would be unacceptable within a year.

The reference to any prior art in this specification is not, and shouldnot be taken as, an acknowledgment or any form of suggestion that suchprior art forms part of the common general knowledge.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a pressure sensor forsensing a fluid pressure, the pressure sensor comprising:

a first wafer substrate with a front side and an opposing back side, achamber partially defined by a flexible membrane formed on the frontside and at least one hole etched from the back side to the chamber;

a second wafer on the back side of the first wafer to seal the at leastone hole; wherein,

the chamber contains a fluid at a reference pressure, such that theflexible membrane deflects due to pressure differentials between thereference pressure and the fluid pressure; and,

associated circuitry for converting the deflection of the flexiblemembrane into an output signal indicative of the fluid pressure;wherein, the second wafer is wafer bonded to the first wafer substrate.

Wafer bonding offers an effective non-adhesive solution. It provides ahermetic seal with only minor changes to the fabrication procedure.Skilled workers in this field will readily understand that the mostprevalent forms of wafer bonding are:

direct wafer, or silicon fusion, bonding;

anodic, or electrostatic Mallory process bonding; and,

intermediate layer bonding.

These forms of wafer bonding are discussed in detail below, and they allavoid the unacceptable air permeability associated with adhesive andpolymer coatings.

A first related aspect provides a pressure sensor for sensing a fluidpressure, the pressure sensor comprising:

a chamber partially defined by a flexible membrane, the chambercontaining a fluid at a reference pressure, such that the flexiblemembrane deflects from any pressure difference between the referencepressure and the fluid pressure, the membrane being at least partiallyformed from conductive material; and,

associated circuitry for converting the deflection of the flexiblemembrane into an output signal indicative of the fluid pressure;wherein,

the chamber, the flexible membrane and the associated circuitry areformed on and through a wafer substrate using lithographically maskedetching and deposition techniques.

The lithographically masked etching and deposition techniques used inthe semiconductor chip manufacturing industry can produce many separatedevices from a single wafer with high yields and low defect rates.Applying these fabrication techniques to MEMS pressure sensors andassociated CMOS circuitry allows high volumes and high yields thatdramatically reduce the unit cost of individual sensors.

A second related aspect provides a pressure sensor for sensing a fluidpressure, the pressure sensor comprising:

a chamber partially defined by a flexible membrane, the chambercontaining a fluid at a reference pressure, such that the flexiblemembrane deflects due to pressure differentials between the referencepressure and the fluid pressure; and,

associated circuitry for converting the deflection of the flexiblemembrane into an output signal indicative of the fluid pressure;wherein, the membrane is less than 0.1 grams.

Designing and fabricating the sensor to minimize the mass of themembrane decreases the effects of acceleration on the membranedeflection. At the same time, a low mass has no effect on the membranedeflection from the pressure differential between the reference fluidand the air pressure.

A third related aspect provides a pressure sensor for sensing a fluidpressure, the pressure sensor comprising:

first and second chambers having first and second flexible membranesrespectively, the first and second flexible membranes configured todeflect in response to pressure differences within the first and secondchambers respectively, the first membrane arranged for exposure to thefluid pressure and the second membrane sealed from the fluid pressure;and, associated circuitry for converting the deflection of the firstflexible membrane into an output signal related to the fluid pressure,and converting the deflection of the second membrane into an adjustmentof the output signal to compensate for the temperature of the sensor.

By sealing the second chamber from the tire pressure, the deflection ofthe second membrane can be determined as a function of temperature. Thiscan be used to calibrate the output signal from the first chamber toremove the effects of temperature variation.

A fourth related aspect provides a pressure sensor for sensing a fluidpressure, the pressure sensor comprising:

a chamber partially defined by a flexible membrane, the chambercontaining a fluid at a reference pressure, such that the flexiblemembrane deflects due to pressure differentials between the referencepressure and the fluid pressure; and,

associated circuitry for converting the deflection of the flexiblemembrane into an output signal indicative of the fluid pressure;wherein, the membrane is at least partially formed from a conductiveceramic material.

Conductive ceramics, such as metal ceramics, have previously been usedto coat tool steels because of its corrosion and wear resistance.Surprisingly, it can be deposited as a thin membrane with sufficientflexibility for sensing pressure while retaining its corrosion and wearresistance. Furthermore, these materials are generally well suited tomicro fabrication processes and electrically conductive so they can beused in capacitative and resistive type pressure sensors.

A fifth related aspect provides a pressure sensor for sensing a fluidpressure, the pressure sensor comprising:

a chamber partially defined by a flexible membrane, the chambercontaining a fluid at a reference pressure, such that the flexiblemembrane deflects due to pressure differentials between the referencepressure and the fluid pressure; and,

associated circuitry for converting the deflection of the flexiblemembrane into an output signal indicative of the fluid pressure;wherein, the membrane is non-planar.

It is possible to extend the linear range of the pressure-deflectionresponse with a non-planar membrane. Corrugations, a series of raisedannuli or other surface features are an added complexity in thefabrication process but can extend the linear range of the sensor by 1MPa.

A sixth related aspect provides a pressure sensor for sensing a fluidpressure, the pressure sensor comprising:

a chamber partially defined by a flexible membrane, the flexiblemembrane at least partially formed from conductive material, and thechamber containing a fluid at a reference pressure, such that theflexible membrane deflects from any pressure difference between thereference pressure and the fluid pressure;

-   -   a conductive layer within the chamber spaced from the flexible        membrane; and, associated circuitry incorporating the flexible        membrane and the conductive layer; such that,    -   the conductive layer and the flexible membrane form capacitor        electrodes and the deflection of the flexible membrane changes        the capacitance which the associated circuitry converts into an        output signal indicative of the fluid pressure; wherein,    -   the conductive layer is arranged such that deflection of the        membrane towards the conductive layer can displace the fluid        from between the membrane and the conductive layer.

Venting the fluid between the electrodes to the other side of the fixedelectrode, while keeping the chamber sealed, avoids the extreme fluidpressure that cause the squeeze film damping.

A seventh related aspect provides a pressure sensor for sensing a fluidpressure, the pressure sensor comprising:

a chamber partially defined by a flexible membrane, the flexiblemembrane is a laminate having at least two layers wherein at least oneof the layers is at least partially formed from conductive material, andthe chamber containing a fluid at a reference pressure, such that theflexible membrane deflects from any pressure difference between thereference pressure and the fluid pressure; and,

-   -   associated circuitry for converting deflection of the flexible        membrane into an output signal indicative of the fluid pressure.

Forming the membrane from a number of separately deposited layersalleviates internal stress in the membrane. The layers can be differentmaterials specifically selected to withstand harsh environments.

A eighth related aspect provides a method of fabricating pressure sensorfor sensing a fluid pressure, the method of fabrication comprising:

etching a recess in a wafer substrate;

depositing a flexible membrane to cover the recess and define a chambersuch that during use the chamber contains a fluid at a referencepressure and the flexible membrane deflects from a pressure differencebetween the reference pressure and the fluid pressure;

depositing associated circuitry for converting the deflection of theflexible membrane into an output signal indicative of the fluidpressure; and,

depositing an apertured guard over the membrane.

An aspect closely related to the eighth aspect provides a pressuresensor for sensing a fluid pressure, the pressure sensor comprising:

a wafer substrate with a recess;

a flexible membrane covering the recess to define a chamber containing afluid at a reference pressure, such that the flexible membrane deflectsdue to pressure differentials between the reference pressure and thefluid pressure;

associated circuitry for converting the deflection of the flexiblemembrane into an output signal indicative of the fluid pressure; and,

an apertured guard over the membrane formed using lithographicallymasked etching and deposition techniques.

By depositing material over the membrane to form the guard offersgreater time efficiency and accuracy than producing a guard separatelyand securing it over the membrane. Semiconductor etching and depositiontechniques allow highly intricate surface details. The apertures in theguard can be made smaller to exclude more particles from contacting themembrane. The fine tolerances of lithographic deposition permit theguard to be positioned close to the membrane for a more compact overalldesign.

A ninth related aspect provides a pressure sensor comprising:

a chamber partially defined by a flexible membrane, the chambercontaining a fluid at a reference pressure, such that the flexiblemembrane deflects due to pressure differentials between the referencepressure and the fluid pressure, the membrane being at least partiallyformed from conductive material;

a conductive layer within the chamber spaced from the flexible membranesuch that they form opposing electrodes of a capacitor; and,

associated circuitry for converting the deflection of the flexiblemembrane into an output signal indicative of the fluid pressure;wherein,

the conductive layer is less than 50 microns from the membrane in itsundeflected state.

A capacitative pressure sensor with closely spaced electrodes can havesmall surface area electrodes while maintaining enough capacitance forthe required operating range. However, small electrodes reduce the powerconsumption of the sensor which in turn reduces the battery size neededfor the operational life of the sensor.

A tenth related aspect provides a pressure sensor for sensing a fluidpressure, the pressure sensor comprising:

a chamber partially defined by a flexible membrane, the chambercontaining a fluid at a reference pressure, such that the flexiblemembrane deflects from any pressure difference between the referencepressure and the fluid pressure, the membrane being at least partiallyformed from conductive material; and,

associated circuitry for converting the deflection of the flexiblemembrane into an output signal indicative of the fluid pressure;wherein,

the flexible membrane is less than 3 microns thick.

The operational range of the pressure sensor requires the membrane tohave a certain deflection. For a given material, the deflection of themembrane will depend on, inter alia, its area and its thickness.Minimizing the thickness of the membrane allows the use of a high yieldstrength membrane material. A thinner membrane also allows the area ofthe membrane to be reduced. Reducing the area of the membrane reducesthe power consumption and the overall size of the sensor. A high yieldstrength material is better able to withstand the extreme conditionswithin the tire and a compact design can be installed in restrictedspaces such as the valve stem.

An eleventh related aspect provides a pressure sensor for sensing afluid pressure, the pressure sensor comprising:

a chamber partially defined by a flexible membrane, the chambercontaining a fluid at a reference pressure, such that the flexiblemembrane deflects from any pressure difference between the referencepressure and the fluid pressure, the membrane being at least partiallyformed from conductive material; and,

associated circuitry for converting the deflection of the flexiblemembrane into an output signal indicative of the fluid pressure;wherein,

the associated circuitry is adapted to be powered by electromagneticradiation transmitted from a point remote from the sensor.

Beaming energy to the sensor removes the need for long-life batteries,or can be used to supplement or charge the batteries. In either case,the sensor avoids the need for large batteries and is therefore smallenough for installation in the valve stem or valve itself.

Optional and Preferred Features

Preferable and optional features of the various broad aspects of theinvention are set out below. The skilled worker in the field willunderstand that while some of the features described below are optionalfor some of the above broad aspects of the invention, they are essentialto other broad aspects.

Preferably the sensor is powered by radio waves transmitted from aremote source. Preferably the sensor is a capacitative pressure sensorwith a conductive layer within the chamber spaced from the flexiblemembrane such that they form opposing electrodes of a capacitor. In afurther preferred form the conductive layer is less than 50 microns fromthe membrane in its undeflected state.

Preferably, the membrane is circular with a diameter less than 500microns. In a further preferred form the membrane is less than 300microns and in specific embodiments the diameter is 100 microns.

In some preferred embodiments the membrane is approximately 0.5 μmthick. In further embodiments, the membrane is a 100 micron diametercircular film. Preferably the metal ceramic is a metal nitride. Inspecific embodiments, the membrane is titanium nitride, tantalumnitride, and vanadium nitride. The membrane may also be form from mixedmetal nitrides. The mixed metal nitrides may be titanium siliconnitride, tantalum silicon nitride, vanadium silicon nitride, titaniumaluminium silicon nitride, tantalum aluminium silicon nitride and so on.

Preferably, the flexible membrane is a laminate having at least twolayers wherein at least one of the layers is at least partially formedfrom conductive material. The layers within the laminate may be formedfrom the deposition of different metal ceramics. Preferably, the metalceramics are metal nitrides or mixed metal nitrides, such as titaniumnitride, titanium aluminium nitride, tantalum silicon nitride, titaniumaluminium silicon nitride, tantalum aluminium silicon nitride and so on.Layers of the laminate may also be metal such as titanium or vanadium.

In a particularly preferred form, the sensor further comprises a secondchamber with a second membrane, the second chamber being sealed from thefluid pressure and the second membrane deflecting from a predeterminedpressure difference in the second chamber; wherein,

the associated circuitry converts the deflection of the second membraneinto an adjustment of the output signal to compensate for thetemperature of the sensor.

In some embodiments, the sensor is formed on and through a silicon waferusing lithographically masked etching and deposition techniques. In afurther preferred form, the sensor is a capacitative sensor, wherein aconductive layer is deposited in each of the first and second chambersand the first and second flexible membranes are conductive, such that,the conductive layer in the first chamber and the first flexiblemembrane form capacitor electrodes wherein the deflection of the firstflexible membrane changes the capacitance which the associated circuitryconverts to the output signal.

Preferably, there is provided a CMOS layer disposed between the metalliclayer and the substrate. In some embodiments, the sensor is additionallyprovided a passivation layer at least partially deposited over themetallic layer.

Optionally, the pressure sensor is adapted to sense the air pressurewithin a pneumatic tire.

Conveniently, the wafer is a first wafer substrate with a front side andan opposing back side, a recess etched into the front side and at leastone hole etched from the back side to the recess; and the sensor furthercomprises:

a second wafer on the back side of the first wafer to seal the at leastone hole; wherein,

the second is wafer bonded to the wafer substrate.

Optionally, the wafer bonding is direct wafer bonding wherein thecontacting surfaces of the first and second wafers are ultra clean, andactivated by making them hydrophilic or hydrophobic prior to bonding,and then brought into contact at high temperature, preferably around1000° C. Anodic bonding offers another option wherein the contactingsurfaces of the first and second wafers have a large voltage appliedacross them. The wafers may be in a vacuum, air or an inert gas when thebond is formed. Intermediate layer bonding is a third option wherein alayer of low melting point material is applied to one or both of thecontacting surfaces of the first and second wafers so that heat andpressure forms the wafer bond. Preferably the low melting point materialis silicon nitride or titanium nitride. This option avoids the highsurface cleanliness required by direct silicon bonding and the highvoltages required by anodic bonding.

In some embodiments, the membrane is non-planar and preferablycorrugated. In a further preferred form, the flexible corrugatedmembrane has a corrugation factor of 8. In a particularly preferredform, the sensor has a linear response up to about 1 MPa. In specificembodiments, the membrane corrugations have a period of between 5microns and 15 microns, preferably about 10 microns. In some preferredembodiments, the membrane is substantially circular and the corrugationsare annular. In a particularly preferred form, the corrugations have asubstantially square-shaped cross-sectional profile.

Preferably the sensor further comprises an apertured guard over themembrane formed using lithographically masked etching and depositiontechniques. In these embodiments, the guard is laminate having at leasttwo layers. In a particularly preferred form the layers are differentmaterials such as silicon nitride and silicon dioxide.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the present invention is now described by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 (Priorart—http://www.pacific-ind.com/eng/products2/carsystem.html) illustratesa typical TPMS used in a four wheel vehicle;

FIG. 2 is a schematic partial section of a sensor according to oneembodiment of the present invention;

FIGS. 3, 5, 8, 10, 13, 15, 18 and 20 are schematic plan views of maskssuitable for use in particular stages of the fabrication process;

FIGS. 4, 6, 7, 9, 11, 12, 14, 16, 17, 19, 21, 22 and 23 illustratesections of an embodiment at successive stages of fabrication;

FIG. 24 is a schematic partial section of a reference sensor forproviding temperature compensation for a sensor according to presentinvention;

FIGS. 25 a to 25 d are a schematic representation of the installation ofthe sensor in the valve stem of a vehicle tire;

FIG. 26 is a diagrammatic representation of the sensor circuit;

FIGS. 27 a to 27 h are partial perspectives (some partially sectioned)of a sensor according to an embodiment of the present invention;

FIGS. 28 a (perspective view) and 28 b (side view) illustrate aschematic of a membrane with corrugations according to a possibleembodiment of the present invention;

FIG. 29 illustrates a comparison of linear and non-linear theory withlinear and non-linear finite element modelling for a flat circulartitanium nitride (TiN) membrane, with R=50 μm and t=0.5 μm;

FIG. 30 illustrates the effect of membrane corrugations onpressure-deflection sensitivity;

FIG. 31 illustrates an expanded view of Von Mises Stress distributionnear an edge of a membrane, with p=1 MPa, q=4 and l=10 μm; and

FIG. 32 illustrates the effect of temperature on the pressure-deflectionresponse of a membrane with q=8 and l=10 μm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following embodiments are described in order to provide a moreprecise understanding of the subject matter of the present invention.While the embodiments focus on a capacitative type sensor, ordinaryworkers in this field will readily understand that the invention isequally applicable to other forms of pressure sensor such as:

-   -   (i) Piezo-resistive, where the membrane is formed from a        non-conductive material and the piezo material is in contact        with the membrane. Deflections of the membrane give rise to        piezo-induced changes in resistivity (and hence current, if a        voltage is applied) that can be monitored electronically.    -   (ii) Resonant pressure sensors, where the frequency of        oscillation of the membrane depends on the pressure difference.        The initial resonance could be activated by using a time-varying        electrostatic force between the two electrodes.    -   (iii) Force compensation pressure sensors, where an        electrostatic force is applied to maintain the membrane at the        initial position. Once again the electrostatic force between the        two electrodes can be used to position the membrane.

Each of these sensor types has particular advantages and limitations.Piezo-resistive sensors are reasonably well known and understood butrequire the use of exotic materials. The sensors of (ii) and (iii) areless popular but do not require exotic materials. Capacitative sensorsare typically robust and highly versatile and therefore the preferredembodiments will be based on this type of sensor.

Functional Overview

A brief overview of the basic operation of the sensor will be describedwith reference to FIG. 2. FIG. 2 shows a schematic partial section of acapacitative sensor fabricated using the masked lithographic etching anddeposition techniques typically used in the production of semiconductorchips. The fabrication steps are described in detail below.

The sensor 30 is formed on a silicon substrate 32, provided with sealedchannels or holes 33, on which is deposited a CMOS layer 34. Aconductive layer 36 is deposited on the CMOS layer 34 followed by apassivation layer 37 as illustrated. The passivation layer 37 may be aninsulating or semi-conducting material. A conductive membrane 50 isspaced from conductive layer 36 to form a reference chamber 58. Roof orcap 54 covers the membrane 54. The roof or cap 54 is provided with holes40, or channels or the like, so that the membrane 50 is exposed to tirepressure.

The membrane 50 deflects due to differential stresses. The amount ofdeflection for a given pressure difference, depends on the membranediameter and thickness, the nature of the support (for example, stronglyfixed, weakly pinned), and the membrane material properties (for exampleelastic modulus, Poisson ratio, density).

Both the membrane 50 and conductive layer 36 are electrodes, whichdevelop a capacitance, C, between them which depends upon the electricalpermittivity of the ambient material, e, the electrode spacing, d, andthe electrode area, A. For the case where both electrodes are circulardisks, C=e A/d. The sensor is then calibrated for measured capacitanceversus applied pressure.

Fabrication Overview

FIGS. 4, 6, 7, 9, 11, 12, 14, 16, 17, 19, 21, 22 and 23 show the mainlithographic etching and deposition steps involved in the fabrication ofa pressure sensor according to the invention. The masks associated withthe successive steps are shown in FIGS. 3, 5, 8, 10, 13, 15, 18 and 20.When etching photoresistive material the solid black areas of the masksare the regions that are removed by the subsequent etch. However, whenetching metal and other non-photoresistive layers, the blank or unmaskedareas of the mask denote the regions that are removed. Skilled workersin this field will understand which masks are applied to photoresist andwhich are applied to non-photoresist.

FIG. 4 is section A-A′ through the wafer 32 of a partially fabricatedsensor. The silicon wafer 32 has previously been ground to the requiredthickness and CMOS circuitry 34 is deposited on its top surface. Thefinal CMOS layer provides the bottom electrode 36 for the sensor. Guardrings 42 are formed in the metallization layers of the CMOS circuitry34. The guard rings 42 prevent air or other fluid from diffusing fromthe subsequently etched sealed passages 33 (see FIG. 2) through thewafer 32 to the circuitry where it can cause corrosion. The mask 70 forthis first metal layer 36 is shown in FIG. 3, with the blank regionsbeing etched away.

As shown in FIG. 6, a passivation layer 37 and sacrificial layer 38 aredeposited next. This is followed by masking and etching through to thesilicon substrate 32. This etch is known as the dielectric etch and theassociated mask 72 is shown in FIG. 5. The mask represents the regions46 that are etched. Following the dielectric etch, the sacrificial layer38 is etched away with a different etchant which also etches the holes33 deeper into the wafer substrate 32 (see FIG. 7).

Referring to FIGS. 8 and 9, the passivation layer 37 is then etched in aregion 44 above the upper contact to provide an electrical pathway forthe second electrode (subsequently deposited).

As shown in FIG. 11, sacrificial material 48 is then deposited to fillthe openings 33 into the silicon 32 made by the dielectric etch. Thisdeposition continues until the top of the sacrificial layer 48 is leveland at a height which provides the requisite gap height between the topand bottom electrodes. The first sacrificial layer 48 is then patternedand etched. The associated mask 76 is shown in FIG. 10.

FIG. 12 shows the deposition of the upper electrode layer 50, alsocalled the second metal layer. This layer is etched with the mask 78shown in FIG. 13, with the blank regions being removed by the etch (seeFIG. 14). The upper electrode layer 50 becomes the flexible membrane inthe finished sensor.

A second sacrificial layer 52 is then deposited (see FIG. 16), andsubsequently etched. The relevant mask 80 is shown in FIG. 15.

FIG. 17 shows the deposition of the roof layer 54. It is then etchedusing mask 82 shown in FIG. 18, with blank areas removed (see FIG. 19).

The wafer is subsequently turned over for ‘back etching’ from thereverse side of the wafer 32.

FIG. 21 shows a deep back etch 56 extending through to meet the openings33 in accordance with the mask 84 shown in FIG. 20. The openings 33 arefilled with sacrificial material 48 which is exposed by the deep backetch 56. The sacrificial material is removed by plasma cleaning (seeFIG. 22) through the deep etch 56.

As shown in FIG. 23, the wafer is again turned over and the sacrificialmaterial 52 is removed through the hole in the roof layer 54. Tocomplete the device, it needs to be packaged, with the bottom face ofthe wafer being sealed. Skilled workers will appreciate that there arevarious methods of achieving this. However, in the preferred embodiment,the bottom face of the wafer is sealed using wafer bonding, which isdiscussed in detail below.

Temperature Compensation

Differential thermal expansion of the components within the pressuresensor will affect the membrane deflection and therefore the sensoroutput. If the sensor is used in an environment with a large temperaturevariation, the sensor accuracy can become unacceptable. To address this,the pressure sensor can be coupled with a temperature sensor, with thepressure sensor then calibrated as a function of temperature.

To accommodate this, an embodiment of the present invention canconveniently incorporate a temperature sensor to account for temperatureeffects on the pressure sensor. A schematic section of the temperatureis shown in FIG. 24. Reference chamber 58, can be etched into the samewafer substrate, but is not exposed to tire pressure like the adjacentpressure sensor (not shown). In these embodiments, the coupled sensorsform an active and a reference sensor, the latter responding only tothermal changes. Skilled workers in this field will appreciate that thereference sensor can also serve as a temperature sensor with theaddition of circuitry calibrating the capacitance to the temperature.

Referring to FIG. 24, the reference sensor is made in the same way asthe active sensor, except that the holes 40 are made in the membrane 50instead of the roof layer 54. The sacrificial material 52 between themembrane 50 and the roof layer 54 is removed with a back etch throughthe holes 40 in the membrane 50.

An alternative to this is to keep the membrane 50 intact and etch awaythe second sacrificial layer material 52 from above the active part ofthis layer, before deposition of the roof layer 54. This causes themembrane 50 to be bonded to the roof 54, and this configuration is muchstiffer. Therefore, the exact dimensions of the reference sensor wouldneed to be adjusted to provide a similar capacitance change in activeand reference sensors due to thermally induced stress changes in themembrane 50.

Temperature Compensating Sensor Design

FIGS. 27 a to 27 h show perspectives of temperature compensating sensorat various stages of fabrication. As best shown in FIGS. 27( a) to27(d), the reference sensor 51 is etched into the same wafer substrate32 as the active sensor 53. This embodiment has further increased thestructural strength by adding the top cover 60 over the roof layers 54of the active and reference membranes. The cover 60 defines separatechambers 62 and 64 over the reference and active roofs 54 respectively.In this embodiment, the roof layers 54 of each sensor stop smallerparticles from contacting the membranes 50. The top cover 60 providesmuch greater structural rigidity while protecting the membrane and roofguard layers 54 from damaging contact during installation. However, evenwith the top cover 60, the sensor has overall dimensions that are smallenough for installation in the tire valve or valve stem.

As best shown in FIG. 27 c, chamber 62 is sealed from the tire pressure,whereas chamber 64 is exposed to the tire pressure via vent 66 andchannel 68. While it has not been shown in the figures, it will beappreciated that the vent 66 extends through to the back surface of thewafer substrate 32, where it is not sealed, but open to the tirepressure. If the sensor is wafer bonded to a sealing wafer (as discussedbelow), the sealing wafer has corresponding holes for establishing afluid connection with the tire pressure.

Semiconductor Fabrication Techniques

Using the lithographically masked etching and deposition procedures ofsemiconductor fabrication, it is possible to manufacture a robust, lowcost tire pressure sensor from Micro-Electro-Mechanical (MEM) baseddevices for use in a TPMS. The membrane can be formed from a materialthat is capable of withstanding a wide range of environmentalconditions. An advantage of such a tire pressure sensor is therelatively low cost of manufacture. The membrane can be formed in manypossible geometries, for example, as a generally flat or planar shapesuch as a disc, or having a featured surface.

Sensor Circuitry

FIG. 26 is a diagrammatic representation of a capacitance sensingcircuit where:

C_(s) is the capacitance of the sensor capacitor;

C_(r) is the capacitance of the reference capacitor (preferably made inthe same way as sensor capacitor but non-sensing); and

C_(p) is a parasitic capacitance to ground.

V1 and V2 are constant voltages reference voltages provided to switches80. These may be chosen to be two of the circuit power supplies orinternal regulated supplies. These voltages are then switched onto oneof each of the capacitor plates charging or discharging them. Thevoltage V_(in) is fed to charge amplifier 84, a high gain device, whichamplifies the voltage to V_(out). V_(in) provides a measure of chargeimbalance in the circuit when it is operated as follows:

Step 1.

Connect C_(r) to V2, Cs to V1; reset the charge amplifier 84 whichforces V_(in) to a fixed voltage V_(r) and set the charge injector to aknow state with charge Q_(I1).

The total stored charge, Q₁, is:Q ₁ =C _(r)(V2−V _(r))+C _(s)(V1−V _(r))+Q _(I1)Step 2.

Connect C_(r) to V1, C_(s) to V2, and remove the reset from the chargeamplifier 84. The output from the charge amplifier 84 is monitored bythe control 86 and feedback applied to the charge injector such that thetotal charge is balanced forcing V_(in)=V_(r) by injecting a charge ofQ_(I1)−Q_(I2).

The total stored charge, Q₂, is given by:Q ₂ =Cr(V1−V _(r))+C _(s)(V2−V _(r))+Q _(I2)

Feedback forces Q₁=Q₂, so that the digital output from the control 86is:Q _(I1) −Q _(I2)=(V ₁ −V ₂)·(C _(r) −C _(s))

The control logic 86 may operate an iterative procedure to determine therequired output to obtain this charge difference, at the end of which itwill produce the required digital output.

The voltage on C_(p) is the same at the end of Step 1 and Step 2, and soideally, does not contribute to the digital output.

Optionally, these steps can be repeated and an averaging applied to thedigital output to reduce random noise. Furthermore, additional steps maybe added to the above idealized case, in order to improve accuracy ofthe circuit.

Sensor Installation and Power Supply

FIGS. 25 a to 25 d schematically show the installation of the sensorwithin the valve stem of a car tire. The pressure sensor can be mountedin other locations including the valve head, the tire wall, the wheelhub and so on. However, the relatively solid structure of the valve stemmakes it the most convenient component for automated installation of thesensor. Furthermore as the stem closer to the centre of the wheel thanthe rest of the tire, the acceleration forces on the sensor generated bywheel rotation are less.

As best shown in FIGS. 25 b and 25 c, the sensor 96 is mounted in thevalve stem 92. A small recess is created in the valve stem wall 94. Athin layer of hard-setting adhesive 98 is applied as a coating on therecess walls. The sensor chip 96 is then adhered into the recess withany excess adhesive removed before the adhesive 98 is cured.

Power can be supplied to the sensor chip 96 in a number of fashions,including, but not limited to, a long-life battery (not shown) locatedin the valve stem wall 94, a long-life battery located in the valve head92, or radio frequency energy beamed to an electromagnetic transducerfrom an external station. The embodiment shown in FIGS. 25 a to 25 d isthe latter. The pressure and temperature are sampled once per second, orat any other rate as required by legal or commercial obligations, andthe results are displayed on the dashboard tire sensor display 106,marked as level monitors 104. If the tire pressure is outside the levelsspecified by the tire manufacturer for proper tire functioning, orindeed any other limits which arise from legal or commercialobligations, the specific tire 90, or tires, will have an error shown incolour (eg. red) on the car chassis symbol 102.

If the power is supplied to the sensors 96 by long-life batteries, thesensor display 106 would include a low battery indicator. This can beconveniently by illuminating the problem signal 102 in a differentcolour (eg. purple).

Low Mass/Conductive Ceramic Membrane

In a particular embodiment, the sensor has a membrane that is at leastpartially a conductive-ceramic compound, for example, titanium nitride,TiN. The use of MEMS-based sensors reduces the effects of accelerationdue to the greatly decreased mass. As an illustrative but non-limitingexample, a TiN membrane, with density of 5450 kgm⁻³, radius of 50 μm andthickness of 0.5 μm, should experience a force of 0.2 μN due to anacceleration of 1000 g; compared with a force of 1.6 mN for a pressureof 207 kPa (approximately 30 psi), which is typical for standard tireinflation pressure. The low mass of the membrane ensures that the affectof acceleration is negligible compared to that of the pressure.

TiN has been found to have a surprisingly high yield strength comparedwith other known materials used for capacitive sensor membranes. Thismakes it suitable for use in a wider range of stressful, harmful ordangerous environments. This also means that under standard conditions,membranes made from TiN should have greater lifetimes compared withstandard capacitive pressure sensors.

The sensor membrane may be composed of other conductive-ceramiccompounds. In particular, metal-ceramics, for example titanium aluminiumnitride (TiAlN), titanium aluminium silicon nitride (TiAlSiN), tantalumnitride (TaN), tantalum aluminium nitride (TaAlN) or the like, havesuitable properties. These metal ceramics are hard wearing and form atough, thin surface oxide. The protective oxide gives the sensors goodrobustness and longevity. Metal ceramics are well suited to depositionby semiconductor fabrication techniques. This allows the sensor to havea thin membrane (0.5 μm to 5 μm) with diameters ranging from about 20 μmto 2000 μm. As discussed below, thin membranes have less internalstresses from rapid cooling, and less mass for limiting accelerationeffects.

Squeeze Film Damping

According to another possible embodiment, an electrode in the sensor canbe provided with holes to prevent squeeze film damping. Squeeze filmdamping occurs when the membrane deflects very close to the staticelectrode. The pressure of the fluid between the membrane and the staticelectrode rises and restricts, or damps, the membrane motion. This canseverely restrict the dynamic response of the membrane.

Channels or apertures can let the fluid between the fixed electrode andthe dynamic electrode (membrane) flow away from the narrowing spacebetween them. The fluid must still remained sealed within the sensor,however, letting it escape from between the opposing faces of themembrane and the fixed electrode avoids squeeze film damping effects.

Internal Stresses

Residual stress in the membrane can affect its deflection and thereforethe accuracy of the sensor. The fabrication process is a typical causeof residual stress. Rapid cooling material from an elevated temperature,can generate thermal stresses in proportion to the material co-efficientof thermal expansion. These stresses can deform the membrane and changeits deflection characteristics under fluid pressure. This in turn canaffect the accuracy of the pressure reading.

As discussed above, masked lithographic deposition of theconductive-ceramic membrane allows it to be very thin—typically of theorder of 0.5 μm thick. The temperature profile across the thickness of athin membrane is much flatter than that of a relatively thick membrane.The membrane cooling is far more uniform and the internal stresses arelargely eliminated.

Laminated Membrane

Masked lithographic deposition also allows the sensor to have arelatively thick membrane for harsh operating conditions, while stillavoiding the problems of residual thermal stresses. By forming themembrane as a laminate, the separately deposited layers are individuallythin enough to avoid residual stress but the final laminate issufficiently strong.

Instead of depositing layers of the same conductive material, theindividual layers can be selected so their collective properties providegood resistance to harsh environments. For example alternate layers ofTiN or TiAlN or various other combinations of metals and ceramics, forexample, Ti and TiN.

Laminated Roof Layer

Likewise the roof layer 54 may also composed of several differentmaterial layers, for example, silicon nitride, Si₃N₄, and silicondioxide, SiO₂. Again, this avoids residual thermal stresses.

Simulation Results and Analysis

To permit an examination of the performance of a TiN membrane capacitivesensor, a series of numerical simulations have been performed using acommercial finite-element modelling package called ANSYS 5.7(http://www.ansys.com). Axisymmetric models were used to reducecomputational time. This required a symmetry boundary condition at thecentre of the membrane, whilst the edge of the membrane was held fixed.A square mesh with one thousand nodes distributed equally across theradius was employed, and meshes with half the number of cells exhibitedless than 5% difference in the maximum stress and deflection.

As discussed above, a MEMS fabrication procedure can be used to deposita 0.5 μm thick layer of TiN to form a membrane. A membrane deflection of5 μm provides sufficient variation in capacitance. For a standardpassenger vehicle, the pressure applied to the membrane is typically bein the range 0-45 psi, allowing for 50% tire over-inflation. Accordingto linear theory (see Young, W. C. and Budynas, R. G., Roark's Formulasfor Stress and Strain, 7^(th) Edition, p 488, 2002), the membraneradius, R, for a specific deflection, Δ, and applied pressure, P, isgiven by:

$\begin{matrix}{R = \left\{ \frac{16\;\Delta\;{Et}^{3}}{3\;{P\left( {1 - v^{2}} \right)}} \right\}^{1/4}} & (1)\end{matrix}$where E is the modulus (approximately 500 GPa for TiN), t is themembrane thickness and v is Poisson's ratio (0.25 for TiN). For thesevalues it is found, from equation (1), that R≈50 μm. The variation ofmembrane deflection with applied pressure is shown in FIG. 3 for amembrane radius of 50 μm. The comparison between linear finite elementmodel (FEM) and linear theory is quite good.

At very high pressure, the deflection-pressure response becomesnon-linear, and it is important to include these non-linear effects whendesigning a working tire pressure sensor.

Non-Planar Membrane

It is possible to extend the linear range of the device by corrugatingthe membrane. FIGS. 28 a and 28 b illustrate a possible embodiment of anon-planar membrane 50 for use in a TPMS sensor. The membrane 50 isgenerally circular in extent and is provided with corrugations formed asannular ridges 22 on a base region 24. The number and spacing of annularridges, and the individual shape of the annular ridges 22, can vary.Also, differently shaped annular ridges 22 could be provided on a singlebase region 24. In the particular embodiment illustrated, thecorrugations are formed to have a square-shaped cross-sectional profile.Geometric parameters s, l, H and t are also illustrated and arereferenced in the following equations. For a circular membrane, thisamounts to superimposing a series of raised annuli on the membraneprofile as illustrated in FIG. 28 b. A theoretical model for thenon-linear response for a corrugated membrane is:

$\begin{matrix}{\frac{{PR}^{4}}{{Et}^{4}} = {{A_{p}\left( \frac{\Delta}{t} \right)} + {{B_{p}\left( \frac{\Delta}{t} \right)}^{3}\mspace{14mu}{where}}}} & (2) \\{A_{p} = \frac{2\left( {q + 2} \right)\left( {q + 1} \right)}{3\left\{ {1 - \left( {v/q} \right)^{2}} \right\}}} & (3) \\{B_{p} = {\frac{32}{q^{2} - 9}\left( {\frac{1}{6} - \frac{3 - v}{\left( {q - v} \right)\left( {q + 3} \right)}} \right)\mspace{14mu}{and}}} & (4) \\{q = {\frac{s}{l}\left\{ {1 + {1.5\left( \frac{H}{t} \right)^{2}}} \right\}}} & (5)\end{matrix}$The variable q is referred to as the corrugation quality factor, s isthe corrugation arc length, l is the corrugation period and h is thecorrugation height (refer to FIG. 28 b). For right-angled corrugations,s=l+2 h. For a flat membrane, q=1. To include non-linear effects in thefinite element calculation, the load is applied over a number ofsub-steps and an equilibrium solution is sought for each sub-step. Theresults for the non-linear simulation and theory are also shown in FIG.29. The response becomes non-linear at approximately 30 kPa, which iswell below the maximum expected tire pressure. The non-linear finiteelement simulations match the linear and non-linear theories below andabove the critical point, respectively.

To assess the effect of corrugations on sensor designs, finite elementmodels were constructed for two different corrugation periods, l=10 and20 μm, and two different quality factors, q=4 and 8. This results in acorrugation height of approximately 0.65 and 1.0 μm for q=4 and 8,respectively. The results in FIG. 30 indicate that it is necessary toinclude non-linear effects for the pressure range considered here;non-linearity should also be present due to the large number of rigidcorners in the model.

The results of the finite element simulations are compared with thetheoretical model in FIG. 30. This shows that a corrugation factor of 8will extend the linearity of the sensor up to an applied pressuredifference of approximately 1 MPa. It also shows that the corrugationperiod does not have a strong effect for the configurations examinedherein.

The maximum Von Mises stress behaves in a similar manner to the membranedeflection. The stress is concentrated near the junction at the lowerside of the outermost corrugation with progressively less stress on theinner corrugations (see FIG. 31 for a typical stress distribution). Thecoefficient of thermal expansion for TiN is 9.4×10⁻⁶ K⁻¹, which meansthat, for the range of likely operating temperatures, thermally inducedstress might alter the sensor deflection. The effect of temperature onthe sensor response is examined in FIG. 32 for a sensor with q=8 andl=10 mm. The range of temperatures T examined, from −20 to +40° C., canbe considered as representative of the heat-up cycle which occurs when avehicle initiates a journey in a cold environment. It is seen that below100 kPa pressure difference, there is a strong effect of temperature. Athigher applied pressures, typical of operation, the thermal stresses areswamped by pressure-induced stresses and the temperature has littleeffect on the sensitivity.

Thus, a corrugated membrane sensor has a linear response in the regionof interest. The small size of the sensor means that it is suitable forinstallation in wheel hub/rim valve stem and valve cap systems.

Wafer Bonding

As discussed above in relation to FIGS. 3 to 23, the deep etch hole 56needs to be sealed to maintain the fluid beneath the membrane 50 at areference pressure. Typically, the reverse side of the wafer 32 isbonded to a sealing wafer.

Unfortunately, simply using a polymeric adhesive to bond a main wafer tothe sealing wafer is not sufficient. The reference cavity (i.e. fluidbeneath the membrane 50) seeks to maintain a constant pressure to ensurea minimum of calibration drift. The polymeric adhesive is permeable toair, which may result in leakage of air into the reference cavitybecause of the tire pressure.

The flow rate across a permeable material is given by:

$\begin{matrix}{Q = \frac{P_{12}A\;\Delta\; P}{L}} & (6)\end{matrix}$where P₁₂ is the permeability of the material, A is the surface area ofthe material exposed to the pressure difference, ΔP is the pressuredifference and L is the flow path length. The permeability of mostpolymers is of order 10⁻²¹ m³ (STP) ms⁻¹ m⁻² Pa⁻¹, so for a pressuredifference=300 kPa, cavity radius=50 μm, seal height=10 μm and seal(flow) length=10 μm, the flow rate is approximately 9.4×10⁻²⁰ m³ s⁻¹.

If the cavity height is 100 μm, then the total cavity volume is7.9×10⁻¹³ m³, and approximately 100 days would be required for thereference cavity pressure to equilibrate with the tire pressure. Whilstthis may be suitable for testing purposes, it would make such a deviceunsuited to practical use.

Another way to seal the reverse side of the wafer is to over-mold thesensor with plastic to increase the path length for leakage. Thepermeability of polymers is about ten times less than that of adhesives,and so a ten-year seal would require a seal length of approximately 0.5mm. This is too long for a MEMS device that is less than 50 m long. Tomaintain the high yield and versatility of the pressure sensor as a MEMSdevice, a different sealing solution is required.

Wafer bonding offers the possibility of a hermetic seal at the cost of aslightly different fabrication procedure. The most prevalent forms ofwafer bonding are: direct wafer bonding, anodic (electrostatic) bondingand intermediate layer bonding (see Table 3).

TABLE 3 Wafter bonding techniques. T_(bond) Surface Type of BondDescription (° C.) Finish Direct Wafer Reactive surfaces brought800-1100 <0.01 μm (Si Fusion) into contact under high temperature AnodicHigh voltage (0.5 to 1.5 kV) 200-1000   <1 μm (Electrostatic, appliedacross both wafers Mallory Process) Intermediate Low melting temperature   1 μm Layer Au—Si material applied to one or 363 Glass Frit both wafersurfaces, bonds 400-600  (e.g. Corning form when temperature and #75xx)Au—Au pressure are applied <400 BCB* 250 PDMS 25 (plasma treated) Tin,Si₃N₄ 100-300? LPCVD PSG 1100 APCVD BO 450 Boron-Doped Si 450 Organics*<200 BCB = benozcyclobutene, PDMS = polydimethylsiloxane, PSG =phosphosilicate glass. *= unsuitable due to porosity.

In the first method, two ultra clean surfaces are brought into contactafter they have been activated (e.g. the surfaces are made hydrophilicor hydrophobic); the bond forms at elevated temperatures (near 1000°C.). In the second, the two wafers are brought into contact, in eithervacuum, air or an inert atmosphere, and a large voltage is appliedacross the wafers. The bond forms because of migration of ions from onewafer surface to the other. This method has relaxed requirements interms of surface finish, which makes it more suitable for bonding twowafers that have undergone a series of fabrication steps; however, thehigh voltage may damage CMOS layers. The third option employs a layer oflow melting point material that is deposited on one or both wafers onthe contact face. The wafers are brought into contact at moderatetemperatures and the bond forms at the interface once pressure isapplied. There are many different materials that may be used to form theintermediate layer (e.g. Si₃N₄ TiN). The intermediate layer methodovercomes the disadvantage of direct wafer (high cleanliness) and anodic(high voltage) bonding.

The invention may also be said to broadly consist in the parts, elementsand features referred to or indicated herein, individually orcollectively, in any or all combinations of two or more of the parts,elements or features, and wherein specific integers are mentioned hereinwhich have known equivalents in the art to which the invention relates,such known equivalents are deemed to be incorporated herein as ifindividually set forth.

Although the preferred embodiment has been described in detail, itshould be understood that various changes, substitutions, andalterations can be made by one of ordinary skill in the art withoutdeparting from the scope of the present invention.

1. A temperature compensating pressure sensor comprising: a substratehaving sealed channels on which is deposited a CMOS layer; a conductivelayer and a passivation layer deposited on the CMOS layer; a conductiveactive membrane spaced from the conductive layer to form an activechamber; a conductive reference membrane spaced from the conductivelayer to form a sealed reference chamber; and a cap which covers themembranes, said cap exposing the active membrane to an outside fluidpressure, wherein the active membrane deflects due to differentialstresses so that an active capacitance is developed between the activemembrane and the conductive layer depending on the electricalpermittivity e of the fluid, with the reference membrane providing atemperature compensating reference capacitance.
 2. The temperaturecompensating pressure sensor of claim 1, wherein the active membrane andconductive layer are circular disks separated by spacing d and eachhaving area A, so that the active capacitance is given as C=e A/d. 3.The temperature compensating pressure sensor of claim 1, wherein eachchamber includes a perforated roof layer to protect the membranes. 4.The temperature compensating pressure sensor of claim 3, wherein theroof layer is manufactured from a material selected from the groupconsisting of: silicon nitride (Si₃N₄) and silicon dioxide (SiO₂). 5.The temperature compensating pressure sensor of claim 1, wherein the capdefines a vent and channel to expose the active chamber to an outsidepressure.
 6. The temperature compensating pressure sensor of claim 1,wherein the substrate includes a silicon wafer.
 7. The temperaturecompensating pressure sensor of claim 1, which is operatively fitted toa tire.